In general, characteristics required of a separator used in metal/halogen batteries are as follows.
1) The separator has an ionic conductivity, a low resistivity, and functions of reducing self-discharges with the metal and the halogen which occur at both electrode compartments.
2) The separator is a stable membrane which prevents halogen from diffusing particularly at an anode and is not deteriorated by strongly oxidizing halogen.
3) The separator is a membrane which is hardly swollen or deflected and can lengthen the life of the batteries.
4) The production cost of the separator is low.
In the present situation, ion-exchange membranes, fluorinated resin porous membranes and polyolefin porous membranes are used as separators which are considered as those having the characteristics described above. Of these, a separator comprising polyethylenes and fine particulate silica as disclosed in JP-B-5-27233 is inexpensive and excellent in resistance to oxidation.
The separator disclosed in JP-B-5-27233, however, is so poor in thermal resistance that as described in JP-A-62-17945, the separator is cracked by heating in a step of attaching an electrode frame to the separator by injection molding. Moreover, said separator has such a low resistance to stress-cracking that when the separator is used as a separator for a metal/halogen secondary battery for a long period of time, this membrane is cracked.
In order to remove these defects, there has been disclosed a technique of blending an ultra-high-molecular weight polyethylene as described in JP-A-9-231957. However, it cannot be said that this technique imparts sufficiently good electric properties, though the technique improves the thermal resistance and the resistance to stress-cracking.
Furthermore, when a polyolefin-based microporous membrane comprising polyolefins and silica is used as the main component of a separator, its bromine permeability is usually not sufficiently low, so that sufficient coulomb (Ah) efficiency cannot be attained. For solving this problem, there have been disclosed, for example, a means of adjusting the ratio between the numbers of silicon atoms (Si) and carbon atoms (C), which are exposed on the surface including the wall surfaces of the separator, i.e., the ratio Si/C to not less than 0.2 as determined by X-ray photoelectron spectroscopy (XPS), as described in JP-A-1-157071; and a means in which the porosity is adjusted to 45 to 50%, the pore volume to 750 to 850 mm3/g, and the average pore radius to 1.5 to 2.0×102 Å as described in JP-A-2-51877.
Both of the separators disclosed in JP-A-1-157071 and JP-A-2-51877, however, have a large average pore size of 0.03 to 0.04 μm. Moreover, the separator disclosed in JP-A-2-51877 has a small pore volume of 750 to 850 mm3/g and hence a high electric resistance of 0.004 Ω·100 cm2/separator. Thus, this separator is still unsatisfactory in electric properties.